A piano is neutral — every key looks alike. The guitar is opinionated: it's tuned in perfect fourths with one major third wedged between the G and B strings, and that specific geometry makes some harmony fall under the fingers and other harmony fight you. Open strings ring sympathetically — their overtones reinforce the bass — so keys rich in open notes (E, A, D, G) sound bigger and are easier, which is a real reason so much guitar music lives there. The CAGED shapes exist because those five open chords are the only fully-open major triads the standard tuning offers; everything else is those shapes barred and slid. And the honest complication: equal temperament makes the guitar's major thirds sharp — about 14 cents above the pure overtone third — so open-G slide masters like Keith Richards and Sonny Landreth nudge the B string flat to let those big one-finger majors ring in tune with the series. Alternate tunings are arguments too: drop the strings into a chord (open D, open G) and the instrument pre-agrees with one key, trading flexibility for resonance — which is why slide blues and Celtic drone music reach for them. The fretboard isn't a neutral grid. It's a physical theory of which chords belong together, and learning it is learning that theory with your hands.